<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>超时与网络设置</title>
    <style>
        #result{
            width: 200px;
            height: 100px;
            border: solid 1px blue;
        }
    </style>
</head>
<body>
    <button>点击发送请求</button>
    <div id="result"></div>

    <script>
        //获取元素对象
        const btn=document.getElementsByTagName('button')[0];
        const result=document.querySelector('#result');

        btn.addEventListener('click',function(){
            const xhr=new XMLHttpRequest();
            //超时2s设置
            xhr.timeout=2000;
            //超时回调
            xhr.ontimeout=function(){
                alert('网络异常，请稍后重试！！');
            }
            //网络异常回调
            xhr.onerror=function(){
                alert('你的网络似乎开了小差');
            }
            
            xhr.open('GET','http://127.0.0.1:8000/delay');
            xhr.send();
            xhr.onreadystatechange=function(){
                if(xhr.readyState===4){
                    if(xhr.status>=200&&xhr.status<300){
                        result.innerHTML=xhr.response;
                    }
                }
            }
        });
    </script>
</body>
</html>